I had this mutator for a long time on my disk now and finally wrote a readme for it. DoubleJumpUT brings doublejumping from UT2003 to UT nothing more, nothing less. Works on-line, but no botsupport (don't know how to implement it).
Get it from http://home.ict.nl/~mweterin/unreal
Get it from http://home.ict.nl/~mweterin/unreal